Mass medication by the nanny state or sound public health? David Derbyshire looks at plans to add more chemicals to your water

It might be clear, odourless and if you're lucky, tasteless, but the water dripping from your kitchen tap is far from pure.

There's chlorine for a start, added to kill bacteria. Then there are the bugs that survived the chlorine, traces of pesticides, lead, nitrate and a few specks of iron. Just to complete the cocktail, and depending on where you live, you could find yourself pouring out a glass of fluoride.

It's the last of these chemicals that has been making the headlines this month. For the past 40 years, fluoride has been added to the tap water of five million people in an attempt to prevent tooth decay.

The Government and health officials are so convinced by the benefits that they are pushing for a change in the law to permit the first major extension of fluoridation since the 1960s.

The plans are being fought by pressure groups who claim fluoridation discolours children's teeth, and may be linked to bone and kidney disease. Rather than giving parents the choice, the plans are the nanny state in action, it says.

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Some of their web sites pull no punches. One states "there is no scientific evidence to prove the benefits of fluoride. Indeed, all the evidence points incontrovertibly to the harm caused by fluoride to human, plant and animal life.''

Could the pro-fluoridation lobby - a group of government, dentists, health authorities and the World Health Organisation - have got it so wrong?

Fluoridation was introduced to the West Midlands in 1964 and to Newcastle in 1969 after trials in the 1950s. The schemes were extended to parts of Yorkshire, Lincolnshire, Merseyside, Oxford and North West Thames.

Today five million customers drink one part fluoride to one million parts water in these areas. Half a million drink water naturally fluorididated by minerals under ground. Another 2.2 million, mostly living in the East of England, drink natural fluoride between 0.3 and 0.7ppm.

Fluoride prevents tooth decay by changing the structure of enamel in infants, making it more resistant to acid attack. It also encourages the re-mineralisation of teeth and may inhibit enzymes used by bacteria to form acid.

Despite the best efforts of health officials, who argue that a third of health authorities have "unacceptably high" levels of tooth decay, fluoridation has not been extended further. I

n 1985 the Water (Fluoridation) Act permitted water companies to accede to health authority requests for fluoridation, but did not force them. Fearful of legal action, the water companies shied away.

Earlier this month, the Government announced that it would amend the law, giving the final decision to the new strategic health authorities. The Department of Health says there is no question of fluoridating the whole country. But health authorities will push for fluoridation in inner-city areas with the worst tooth decay following "proper consultation".

Opposition is being led by the National Pure Water Association. It says that fluoridation deprives the individual of choice, the supposed benefits may be outweighed by harmful side effects and that evidence for benefits are weak.

Jane Jones, the campaign director, sees fluoridation as unethical "deliberate pollution". The former psychiatric nurse's concerns range from the effect of fluoride on the appearance of children's teeth, to bone disease, kidney problems and cancer. Crucially, she says, fluoridation indiscriminately raises exposure of people who may already have high levels through toothpaste and diet.

"If you are debating going to give people fluoride in the water and you don't even know how they are already exposed to it, how scientific is it to say that one part per million is optimum?"

The British Fluoridation Society, a group representing pro-fluoride dentists, says the strongest evidence for the benefits of fluoride, and the lack of evidence of any harm, comes from the experience of millions of people.

If fluoride was so bad, surely we would have seen clusters of serious disease in the West Midlands or any of the other areas with flouridation. Even more compelling is the evidence from the half a million people who, for generations, have drunk naturally fluoridated water.

But the evidence, the pure water campaigners say, is far from conclusive. In 2000, the NHS Centre for Reviews and Dissemination at York University looked at 214 papers it judged to be of sufficient standard. It concluded that fluoride in water reduced tooth decay, said there was no evidence of cancer, bone or kidney problems and highlighted the risk of dental fluorosis, or mottled teeth.

But it was critical of the quality of research. For instance, there were no randomised controlled trials of the effects of water fluoridation. Many studies were decades old, and carried out before the widespread use of fluoride toothpaste.

The report concluded: "Given the level of interest surrounding the issue of public water fluoridation it is surprising to find that little high quality research has been undertaken.

It added: "The evidence of a benefit of a reduction in caries should be considered together with the increased prevalence of dental fluorosis. The research evidence is of insufficient quality to allow confident statements about other potential harms or whether here is an impact on social inequalities."

Although the evidence suggested a reduction in tooth decay the report was hardly a ringing endorsement. In response to the York research, the Government asked the Medical Research Council to suggest studies to improve the evidence base.

Its 2002 report supported the York findings and called for research into individual exposure levels, differences in the absorption of natural and artificial fluoride, effects on cavities given the widespread use of toothpaste, and a comparison of mottling of teeth in fluoridated and non fluoridated areas. The Department of Health is funding a study, with results due in the autumn.

Mrs Jones says the York study has been misrepresented by the Government and the MRC. She also believes that the MRC study was selective and ignored animal studies and cell studies that raised concerns about fluoride. And, she said, the difference between natural and artificial fluoride could be important.

Much of the pro-fluoridation evidence comes from studies of natural fluoride and she believes the effects of fluoride may be more intense if it is added in the form of hexafluorosilicic acid.

While the pro-fluoridation lobby often play down the problem of dental fluorosis, she believes it is a serious concern that can cause psychological damage in children. The British Fluoridation Society accepts that fluoridation can cause mottling, but says it is "often undetectable".

Severe fluorosis is rare and is a cosmetic, not a health issue. Mottling can be caused by parents giving children fluoride supplements in areas where water is already fluoridated, or when young children swallow large amounts of fluoride toothpaste each day, the society argues.

It says there is no evidence that drinking fluoride at one part per million causes more serious skeletal fluorosis - damage to the bones - or any of the more serious diseases that worry the anti lobby.

Michael Lennon, professor of dental public health at Liverpool University and Fluoridation Society member, wants fluoridation extended from 10 to 30 per cent of the UK. "We not only have 50 years of artificial fluoridation, but also generations of people exposed to natural fluoride," he said.

"We are not asking for fluoridation for everyone, but we want to clarify the legislation so decisions are made by health authorities, not water companies. There are many parts of the country where levels of disease are so low that water fluoridation would not be justified. But in Glasgow, London, Manchester and Leeds, fluoridation could save teeth from decay."

Kevin Prior, an expert in drinking water chemistry for the environmental management consultants CookPrior Associates and member of the Royal Society of Chemistry, said: "To get dental fluorosis you would need to use a fluoride toothpaste and have a lot of fluoride in your diet. You would have to drink 25 litres of fluoridated water a day for life in order to suffer.''

He believes there is no reason why the naturally occurring fluoride should act differently in the body to fluoride in naturally found in water. The official advice may not be backed by the best science, but has come down on one side. In the light of MMR and GM, will that be enough to persuade the public?

Tracey Brown, the director of Sense About Science, a group that studies public attitudes to risk, said campaigners against fluoridation had "failed to make the issue stick", partly because the public health campaigns on tooth decay in the 1960s and 1970s were so effective.

"The scares about fluoride toxicity run counter to too much of our immediate, lived experience to be spontaneously effective," she said.

"But we are living through a difficult period in the Government's relationship with the population over matters of public health. They can't decide how to lead on health questions, backing away from decisions one minute and heavy handed the next.

"With such unbalanced, post BSE politics of public health, it is possible that a few people, combined with one of the newspapers trying its hand at a new campaign, will create enough anxiety in Whitehall for the fluoridation issue to become the next public health farce."

Where is it found?

There is much confusion between fluorine and fluoride. Fluorine is the reactive, deadly element - rarely found in its natural gas form. Fluoride is an ion, a fluorine atom that has gained an extra electron. It is stable and found in rocks, water, plants and animals.

In the blood, fluoride exists at around 0.5 parts per million, while in the bone it's found at anything up to 12,000 parts per million. A typical adult body has between three and six grams of fluoride.

Fluoride is regarded today as essential for people at minute doses. However, five grams would be fatal - it has been used as an insecticide for ants and cockroaches.

The average adult has between 0.3 and three milligrams a day in their diet - from fluoridated water, toothpaste, chicken, tea, eggs, potato, cheese. A cup of tea provides 0.4 milligrams. Cod mackerel, sardines and salmon are good sources because sea water contains around one part per million - the accepted "optimum dose".